The invention relates to a process for producing a cured silsesquioxane resin having high fracture toughness and strength without loss of elastic modulus and glass transition temperature. With more particularity the invention relates to a cured silsequioxane resin having colloidal silica having a surface coating formed thereon dispersed within the silsesquioxane resin.
Silsesquioxane resins have seen increased use in industrial applications in the automotive, aerospace, naval and other manufacturing industries. Silsequioxane resins exhibit excellent heat and fire resistant properties that are desirable for such applications. These properties make the silsesquioxane resins attractive for use in fiber-reinforced composites for electrical laminates, structural use in automotive components, aircraft and naval vessels; Thus, there exists a need for rigid silsesquioxane resins having increased flexural strength, flexural strain, fracture toughness, and fracture energy, without significant loss of modulus or degradation of thermal stability. In addition, rigid silsesquioxane resins have low dielectric constants and are useful as interlayer dielectric materials. Rigid silsesquioxane resins are also useful as abrasion resistant coatings. These applications require that the silsesquioxane resins exhibit high strength and toughness.
Conventional thermoset networks of high cross-link density, such as silsesquioxane resins, typically suffer from the drawback that when measures are taken to improve a mechanical property such as strength, fracture toughness, or modulus, one or more of the other properties suffers a detriment.
Various methods and compositions have been disclosed in the art for improving the mechanical properties of silicone resins including: 1) modifying the silicone resin with a rubber compound, as disclosed in U.S. Pat. No. 5,747,608 which describes a rubber-modified resin and U.S. Pat. No. 5,830,950 which describes a method of making the rubber-modified resin; 2) adding a silicone fluid to a silicone resin as disclosed in U.S. Pat. No. 5,034,061 wherein a silicone resin/fluid polymer is adapted to form a transparent, shatter-resistant coating.
While the above referenced patents offer improvements in the strength of silicone resins, there is an additional need to further improve the strength and toughness of silicone materials for use in high strength applications, such as those described above. There is also a strong need to further increase the strength, toughness, modulus and to raise the glass transition temperature simultaneously.
Therefore, it is an object of this invention to provide a process that may be utilized to prepare a cured silsesquioxane resin having high strength and fracture toughness without loss of modulus and glass transition temperature. It is also an object of the invention to provide a process to prepare a cured silsesquioxane resin having simultaneously increased strength, toughness, modulus with an increased glass transition temperature.
A hydrosilylation reaction curable composition including a silsesquioxane polymer, a cross-linking compound, a hydrosilylation reaction catalyst and colloidal silica having a surface coating formed thereon.
There is also included a process for preparing a hydrosilyation reaction curable composition and producing a cured silsesquioxane resin comprising the steps of:
a) providing a silsesquioxane polymer;
b) providing a cross-linking compound;
c) providing colloidal silica with a surface treatment formed thereon;
d) mixing the components of a), b), c) to form a curable composition;
e) adding a hydrosilylation reaction catalyst to the curable composition of step d)
f) curing the curable composition of step e) to form a cured resin having high fracture toughness and strength without the loss of elastic modulus and glass transition temperature.
This invention relates to a hydrosilylation reaction curable composition and process that is used to prepare a cured silsesquioxane resin. This curable composition comprises: (A) a silsesquioxane copolymer, (B) a silicon hydride containing hydrocarbon, silane or siloxane as a crosslinker, (C) a catalyst, (D) an optional solvent (E) a catalyst inhibitor and (F) colloidal silica having a surface coating of various compositions formed thereon.
Component (A) is a silsesquioxane copolymer comprising units that have the empirical formula R1aR2bR3cSiO(4-a-b-c)/2, wherein: a is zero or a positive number, b is zero or a positive number, c is zero or a positive number, with the provisos that 0.8xe2x89xa6(a+b+c)xe2x89xa63.0 and component (A) has an average of at least 2 R1 groups per molecule, and each R1 is independently selected from monovalent hydrocarbon groups having aliphatic unsaturation, and each R2 and each R3 are independently selected from monovalent hydrocarbon groups and hydrogen. Preferably, R1 is an alkenyl group such as vinyl or allyl. Typically, R2 and R3 are nonfunctional groups selected from the group consisting of alkyl and aryl groups. Suitable alkyl groups. include methyl, ethyl, isopropyl, n-butyl, and isobutyl groups. Suitable aryl groups include phenyl groups. Suitable silsesquioxane copolymers for component (A) are exemplified by (PhSiO3/2)0.75(ViMe2SiO1/2)0.25, where Ph is a phenyl group, Vi represents a vinyl group, and Me represents a methyl group.
Component (B) is a silicon hydride containing hydrocarbon having the general formula HaR1bSiR2SiR1cHd where R1 is a monovalent hydrocarbon group and R2 is a divalent hydrocarbon group and where a and dxe2x89xa71, and a+b=c+d=3. The general formula HaR1bSiR2SiR1cHd although preferred in the present invention is not exclusive of other hydrido silyl compounds that can function as cross-linkers of the component (A). Specifically a formula such as the above, but where R2 is a trivalent hydrocarbon group can also be suitable as component (B). Other options for component (B) can be mixtures of hydrido-silyl compounds as well.
Suitable silicon hydride containing hydrocarbons of component (B) can be prepared by a Grignard reaction process. For example, one method for making a silyl-terminated hydrocarbon for use in this invention includes heating to a temperature of room temperature to 200xc2x0 C., preferably 50xc2x0 C., a combination of magnesium and a solvent such as diethylether or tetrahydrofuran. A di-halogenated hydrocarbon, such as dibromobenzene is then added to the magnesium and solvent over a period of several hours.
After complete addition of the di-halogenated hydrocarbon, a halogenated silane, such as dimethylhydrogenchlorosilane, is then added, and an optional organic solvent can also be added. The resulting mixture is then heated for a period of several hours at a temperature of 50 to 65xc2x0 C. Any excess halogenated silane is then removed by any convenient means, such as neutralization with a saturated aqueous solution of NH4 Cl. The resulting product can then be dried with a drying agent such as magnesium sulfate and then purified by distillation.
An example of such a silicon hydride containing hydrocarbon produced by a Grignard reaction includes p-bis(dimethylsilyl)benzene which is commercially available from Gelest, Inc. of Tullytown, Pa.
Component (B) may also be a silane or siloxane that contain silicon hydride functionalities that will cross-link with the vinyl group of component (A). Examples of suitable silanes and siloxanes that may be utilized as component (B) include di phenylsilane and hexamethyltrisiloxane. Such compounds are commercially available from Gelast, Inc. of Tullytown, Pa. and United Chemical Technologies of Bristol, Pa. Component (B) can also be mixtures of hydrido containing silane and siloxanes.
Components (A) and (B) are added to the composition in amounts such that the molar ratio of silicon bonded hydrogen atoms (SiH) to unsaturated groups (Cxe2x95x90C) (SiH:Cxe2x95x90C) ranges from 1.0:1.0 to 1.5:1.0. Preferably, the ratio is in the range of 1.1:1.0 to 1.5:1.0. If the ratio is less than 1.0:1.0, the properties of the cured silsesquioxane resin will be compromised because curing will be incomplete. The amounts of components (A) and (B) in the composition will depend on the number of Cxe2x95x90C and Sixe2x80x94H groups per molecule. However, the amount of component (A) is typically 50 to 80 weight % of the composition, and the amount of component (B) is typically 2 to 50 weight % of the composition.
Component (C) is a hydrosilylation reaction catalyst. Typically, component (C) is a platinum catalyst added to the composition in an amount sufficient to provide 1 to 10 ppm of platinum based on the weight of the composition. Component (C) is exemplified by platinum catalysts such as chloroplatinic acid, alcohol solutions of chloroplatinic acid, dichlorobis(triphenylphosphine)platinum(II), platinum chloride, platinum oxide, complexes of platinum compounds with unsaturated organic compounds such as olefins, complexes of platinum compounds with organosiloxanes containing unsaturated hydrocarbon groups, such as Karstedts catalyst (i.e. a complex of chloroplatinic acid with 1,3-divinyl-1,1,3,3-tetramethyldisiloxane) and 1,3-diethenyl-1,1,3,3-tetramethyldisiloxane, and complexes of platinum compounds with organosiloxanes, wherein the complexes are embedded in organosiloxane resins. A particularly preferred catalyst is a 0.5% platinum containing platinum-divinyltetramethyldisiloxane complex commercially available from Dow Corning Corporation of Midland, Mich.
Component (D) may include an optional catalyst inhibitor, typically added when a one part composition is prepared. Suitable inhibitors are disclosed in U.S. Pat. No. 3,445,420 to Kookootsedes et al., May 20, 1969, which is hereby incorporated by reference for the purpose of describing catalyst inhibitors. Component (D) is preferably an acetylenic alcohol such as methylbutynol or ethynyl cyclohexanol. Component (D) is more preferably ethynyl cyclohexanol. Other examples of inhibitors include diethyl maleate, diethyl fumamate, bis(2-methoxy-1-methylethyl)maleate, 1-ethynyl-1-cyclohexanol, 3,5-dimethyl-1-hexyn-3-ol, 2-phenyl-3-butyn-2-ol, N,N,Nxe2x80x2,Nxe2x80x2-tetramethylethylenediamine, ethylenediamine, diphenylphosphine, diphenylphosphite, trioctylphosphine, diethylphenylphosphonite, and methyldiphenylphosphinite.
Component (D) is present at 0 to 0.05 weight % of the hydrosilylation reaction curable composition. Component (D) typically represents 0.0001 to 0.05 weight % of the curable composition. Component (D) preferably represents 0.0005 to 0.01 weight percent of the total amount of the curable composition. Component (D) more preferably represents 0.001 to 0.004 weight percent of the total amount of the curable composition.
Components (A), (B), (C) and (D) comprise 10 to 99.9 weight % of the composition. The composition may further comprise one or more optional components such as processing additives or other components known in the art.
The hydrosilylation reaction curable composition comprising components (A), (B), (C) and (D), and any optional components can be dissolved in component (E), an optional solvent. Typically, the amount of solvent is 0 to 90 weight %, preferably 0 to 50 weight % of the curable composition. The solvent can be an alcohol such as methyl, ethyl, isopropyl, and t-butyl alcohol; a ketone such as acetone, methylethyl ketone, and methyl isobutyl ketone; an aromatic hydrocarbon such as benzene, toluene, and xylene; an aliphatic hydrocarbon such as heptane, hexane, and octane; a glycol ether such as propylene glycol methyl ether, dipropylene glycol methyl ether, propylene glycol n-butyl ether, propylene glycol n-propyl ether, and ethylene glycol n-butyl ether; a halogenated hydrocarbon such as dichloromethane, 1,1,1-trichloroethane and methylene chloride; chloroform; dimethyl sulfoxide; dimethyl formamide; acetonitrile and tetrahydrofuran. A preferred solvent is toluene.
The hydrosilylation reaction curable composition further includes component (F), colloidal silica with a surface coating formed thereon. The colloidal silica may comprise from 0.1 to 90 weight percent of the reaction curable composition and is preferably from 20 to 80 weight percent of the reaction curable composition and even more preferably from 5 to 25 weight percent of the reaction curable composition.
The colloidal silica particles may range in size from 5 to 150 nanometers in diameter, with a particularly preferred size of 75 nanometers.
The colloidal silica particles are treated with a surface coating by reacting a siloxane or mixture of siloxanes with the silica particle such that silyl groups are formed on the surface of the silica particle. Suitable reactive silanes or siloxanes can include functionalities such as: vinyl, hydride, allyl, aryl or other unsaturated groups. Particularly preferred siloxanes for use as a surface coating include hexamethyldisiloxane and tetramethyldivinyldisiloxane.
The surface coated silica particles may be formed by mixing silica particles with deionized water to form a solution and then adding concentrated hydrochloric acid, isopropyl alcohol, and a siloxane or mixture of siloxanes. The above mixture is then heated to 70xc2x0 C. and is allowed to stir for 30 min. As the hydrophilic silica becomes hydrophobic due to the silylation of silica surface silanols, the silica phase separates from the aqueous phase. No excess organic solvent is required to induce phase separation, as is often the case. Once separation occurs, the aqueous layer (isopropyl alcohol, water, excess treating agent and HCl) is decanted. Deionized water is added to the decanted mixture to wash the treated silica. This step may be repeated a second time to insure adequate washing. To the washed silica solution, a solvent is added and the mixture is heated to reflux to azeotrope residual water and water-soluble reagents.
There is also disclosed a process for preparing a hydrosilyation reaction curable composition comprising the steps of:
a) providing a silsesquioxane polymer;
b) providing a cross-linking compound;
c) providing colloidal silica with a surface treatment formed thereon;
d) mixing the components of a), b), c) to form a curable composition;
e) adding a hydrosilylation reaction catalyst to the curable composition of step d)
f) adding an optional reaction inhibitor to the catalyst of step e) before or after mixing the reaction catalyst with the curable composition;
g) curing the curable composition of step e) to form a cured resin having high fracture toughness and strength without the loss of elastic modulus and glass transition temperature.
The silsesquioxane polymer, as described previously, is first mixed with the cross-linking compound, as disclosed above, and then the colloidal silica having the surface treatment is added. Either a conventional mixer or a high shear rotor/stator mixer may be utilized by the present invention, although a rotor/stator mixer is preferred due to the increased dispersion of the silica particles in the composition, leading to an increase in the mechanical properties of the cured resin. After the components above are mixed, the hydrosilylation catalyst is mixed into the composition and the mixture is poured into a mold. The mixing of the curable composition of the present invention may also include the step of degassing the composition before curing. Degassing is typically carried out by subjecting the composition to a mild vacuum.
The mold is then subjected to the following curing steps: 1) leaving the curable composition of step f) in a mold at room temperature overnight, 2) curing the curable composition in the mold at a temperature of 60xc2x0 C. for 6 hours, 3) curing the curable composition in the mold at a temperature of 100xc2x0 C. for 2 hours, 4) curing the curable composition in the mold at a temperature of 160xc2x0 C. for 2 hours, 5) curing the curable composition in the mold at a temperature of 200xc2x0 C. for 3 hours, 6) curing the curable composition in the mold at a temperature of 260xc2x0 C. for 6 hours.